CAST IRON WELDING
Cast iron comprises alloys of iron with a carbon content of 2–5%, a silicon content of 1–3% and up to 1% manganese. Cast iron exhibits low ductility, low hardness and low strength and is generally a very brittle material. To improve these properties, cast iron is very often alloyed or heat-treated.
The types of cast iron which are mainly used today are:
The high carbon content affects its weldability considerably. Because of the range of properties of cast iron, weldability varies accordingly. Some types are casually welded, others are unweldable. All the iron grades above are successfully welded, apart from white irons because of their extreme brittleness.
Consumables for cast iron
Pure nickel type
As a guideline, cast iron is welded with pure nickel electrodes. Nickel has a capability to absorb more carbon without changing its own properties. The co-expansion of Nickel and cast iron due to heat is comparable. Nickel is more ductile than other filler materials for cast iron welding and is very easy to machine. It is used for filling cavities, general repairs where a hardness of approx. 150HB is required. It is not recommneded for irons with a high sulphur and phosphorus content.
Nickel-iron type
To obtain still higher strength, nickel-iron electrodes can be used for joining cast iron and cast iron to steel. Due to the ferrous content of the weld metal, there is a slight increase in the hardness of the weld metal, compared with pure nickel weld metal. The weld metal is machinable. The nickel-iron type is more tolerant of dilution with sulphur and phosphorus than the pure nickel type.
Nickel-copper type
When colour-matching weld metal is required, the nickel-copper type is suitable. The weld metal is easily machined.
Unalloyed steel type
This type of electrode can be used for non-critical work and when no machining is required.
Joint preparation for cast iron
Since cast iron has a porous metallurgical structure, it absorbs oil and liquid which affect weldability and must thus be removed before welding. In order to burn out these liquids, heating is required. However, in most cases, this is impossible, due to the shape of the object or because of time limitations. One way to get around this problem is to use the gouging electrode. It is excellent for the preparation of repairs in cast iron as it dries out and burns away impurities/graphite on the surface and thus reduces the risk of cracking and porosity when welding.
With normal grinding, the impurities and graphite in the cast iron are smeared out along the groove and may cause problems when welding.
On some welds, it is beneficial to use the buttering technique. This means that one or both of the surfaces to be welded are clad prior to joining. The technique is used to avoid the formation of brittle phases. The contractional stresses from the cooling weld metal in subsequent beads will have more effect on the ductile buttering layer than the brittle HAZ of the base material.
Cold welding cast iron
Most cast iron repairs are performed using SMAW and nowadays the cold welding technique is mainly used with the following procedure.